1. Field of the Invention
This invention relates to surface hardening of biocompatible metallic metals and alloys, suitable for use as material for a medical implant, including in particular, niobium, titanium, and zirconium based alloys which do not include any elements which have been shown or suggested as having short term or long term potential adverse biological effects. More specifically, this invention relates to medical implants made of these surface-hardened alloys with improved resistance to micro-fretting wear and surface abrasion.
2. Description of the Related Art
The most common materials used for load-bearing medical implants such as orthopedic or cardiovascular implants, are metallic alloys, ceramics, and composites formed from biocompatible polymers and various reinforcing materials.
Metals and metal alloys such as stainless steel, Co-Cr-Mo alloy, titanium, and titanium alloys have been used successfully for many years as implant materials, particularly for orthopedic applications. These materials have the requisite strength characteristics needed for such implants but are susceptible to fretting, wear, and corrosion in the body unless treated to reduce these effects. Particulates produced by joint articulation processes or micromotion between assembled devices tend to cause accelerated wear of prosthetic joints and trauma devices.
Further, concern has been raised about potential abrasion between implant metals and adjacent bone and bone cement. This abrasion creates particulates which are associated with adverse cellular response, including bone cell death and eventual loosening of the implant and subsequent revision.
To prevent micro-fretting of the implant surface, the surface may be coated with an amorphous diamond-like carbon coating or a ceramic-like coating, such as titanium nitride or titanium carbide, using chemical or plasma vapor deposition techniques to provide a hard, impervious, smooth surface coating. These coatings are especially useful if the prosthesis is subjected to conditions of wear, such as, for instance, in the case of bearing surfaces of knee or hip prostheses, or between screws and bone plates or modular implants. For the case of orthopedic implant bearing surfaces, bone cement fragments can abrade these relatively thin (up to about 5 microns) surface coatings with time and create hard, abrasion products from these coatings which in turn further accelerate abrasion and wear of the softer underlying metal substrate.
Methods for providing amorphous diamond-like carbon coatings are known in the art and are disclosed in, for example, EPO patent application 302 717 A1 to Ion Tech and Chemical Abstract 43655P, Vol. 101 describing Japan Kokai 59/851 to Sumitomo Electric. Chemical Abstract 43655P, Volume 101 describes the Sumitomo Electric patent as: "Prosthetics made of Ti, A1, and stellites are coated with a film of diamond or diamond-like C to increase the resistance against wear. Thus, an artificial bone joint was prepd. using pure Ti. This was coated with 1 .mu.-thick diamond-like C, using C.sub.2 H.sub.6 in induction heating. The product was biocompatible and resistant to wear in the rabbit." EPO patent application 302 717 A1 describes methods of producing hard diamond-like coatings by direct beam deposition "from a saddle field source from hydrocarbon precursor gases such as propane, butane, and acetylene." EPO patent application 302 717 A1 also notes that "The hard carbon coating can be produced . . . . by chemical vapour depositions . . ." and "A coating thickness of 500 Angstroms is preferred although thicknesses up to 2000 Angstroms may be provided for increased protection."
With orthopaedic and cardiovascular implants being implanted in younger people and remaining in the human body for longer periods of time, there is a need for an implant material with requisite strength and high abrasion resistance which minimizes the production of abrasive particles from surface abrasion effects.